Binaphthyl-Containing Green- and Red-Emitting ...

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DOI: 10.1002/adfm.200800375

Binaphthyl-Containing Green- and Red-Emitting Molecules for Solution-Processable Organic Light-Emitting Diodes** By Yi Zhou, Qingguo He,* Yi Yang, Haizheng Zhong, Chang He, Guangyi Sang, Wei Liu, Chunhe Yang, Fenglian Bai, and Yongfang Li*

Strong intermolecular interactions usually result in decreases in solubility and fluorescence efficiency of organic molecules. Therefore, amorphous materials are highly pursued when designing solution-processable, electroluminescent organic molecules. In this paper, a non-planar binaphthyl moiety is presented as a way of reducing intermolecular interactions and four binaphthylcontaining molecules (BNCMs): green-emitting BBB and TBT as well as red-emitting BTBTB and TBBBT, are designed and synthesized. The photophysical and electrochemical properties of the molecules are systematically investigated and it is found that TBT, TBBBT, and BTBTB solutions show high photoluminescence (PL) quantum efficiencies of 0.41, 0.54, and 0.48, respectively. Based on the good solubility and amorphous film-forming ability of the synthesized BNCMs, double-layer structured organic light-emitting diodes (OLEDs) with BNCMs as emitting layer and poly(N-vinylcarbazole) (PVK) or a blend of poly[N,N0 -bis(4-butylphenyl)-N,N0 -bis(phenyl)benzidine] and PVK as hole-transporting layer are fabricated by a simple solution spin-coating procedure. Amongst those, the BTBTB based OLED, for example, reaches a high maximum luminance of 8315 cd m2 and a maximum luminous efficiency of 1.95 cd A1 at a low turn-on voltage of 2.2 V. This is one of the best performances of a spin-coated OLED reported so far. In addition, by doping the green and red BNCMs into a blue-emitting host material poly(9,9-dioctylfluorene-2,7-diyl) high performance white light-emitting diodes with pure white light emission and a maximum luminance of 4000 cd m2 are realized.

1. Introduction Organic conjugated molecules have attracted much attention due to their potential application in a number of optoelectronic devices, e.g., organic light emitting diodes (OLEDs), photovoltaic cells, photonic devices, and many more.[1,2] They provide notable advantages, like well-defined molecular structures, facile purification by standard techniques, and specific structure-property correlations.[3–5] However, the strong intermolecular interaction of common conjugated organic molecules makes it extremely difficult to

[*] Prof. Y. F. Li, Y. Zhou, Y. Yang, H. Z. Zhong, Dr. C. He, G. Y. Sang, W. Liu, Dr. C. H. Yang, Prof. F. L. Bai Beijing National Laboratory for Molecular Sciences CAS Key Laboratory of Organic Solids Institute of Chemistry, Chinese Academy of Sciences Beijing, 100190 (PR China) E-mail: [email protected] Y. Zhou, Y. Yang, H. Z. Zhong, G. Y. Sang Graduate University of the Chinese Academy of Sciences Beijing, 100039 (PR China) Dr. Q. G. He Shanghai Institute of Microsystem and Information Technology Chinese Academy of Sciences Shanghai, 200050 (PR China) E-mail: [email protected] [**] This work was supported by the NSFC (Nos. 50633050, 20721061, 50503020, and 20773156).

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dissolve them and even then they tend to form crystalline domains in the film state. These domains, which act as carrier traps, can raise the operational voltage of a device.[6–8] Besides, crystalline domains can also lead to luminescence quenching and a decrease in the device’s stability.[7,8] Therefore, it is of significant importance for future OLEDs applications to design and subsequently synthesize amorphous molecules in high purity that exhibit a high fluorescence quantum efficiency and are highly soluble.[9,10] The non-planar binaphthyl unit, with its large dihedral angle and its twisted conformation, has proved to be a suitable moiety that yields a stable amorphous phase and results in good solubility. To date, binaphthyl-containing polymers,[11–14] dendrimers,[15,16] and small organic molecules[17–19] have already been widely used in chiral sensors,[20–23] unsymmetrical catalysis,[24–27] photochromic application,[28–30] and nonlinear optical materials;[31,32] but in recent years binaphthyl-containing materials have also been successfully employed as blue-emitter in electroluminescent (EL) devices.[11–14,17,18,33–40] Yet, greenand red-emitting materials based on binaphthyl-containing molecules (BNCMs) are rarely reported so far. In this work, we report the synthesis and EL properties of four highly soluble green- and red-emitting BNCMs: BBB, BTBTB, TBBBT, and TBT, which are shown in Scheme 1. Benzothiadizole, an electron acceptor unit, was introduced to decrease the lowest unoccupied molecular orbital (LUMO) levels of the BNCMs and by incorporation of the

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Y. Zhou et al. / Binaphthyl-Containing Green- and Red-Emitting Molecules for OLEDs

B(OH)2 H+

OMe n-BuLi, Et2O B(OMe)3 OMe TMEDA,r.t.

B(OH)2

OMe OMe

OMe OMe

+

B(OH)2

1 N

S

Br

S

4

SnBu3

S

Pd(PPh3)4,THF reflux

N

S

N

N

N

S

5

Br

Br

S

S

N

3

2

N

CHCl3, HOAc r.t.

N

S

N S

NBS

N

S

Br

Br

S

6

N Br

N Pd(OAc)2, NaOAc DMF, TOP, 100oC

7 S N N

B(OH)2

S

Br N

OCH3 OCH3

N S

B(OH)2

S

N2, Pd(PPh3)4, THF, KOH/H2O, refulx, 48 hr

OMe OMe

TBT

POT,18-CROWN-6

N S N

emitting layers (EMLs) during the construction of various high performance OLEDs. Amongst those, the OLED based on BTBTB, for example, exhibits a maximum luminance of 8315 cd m2 and a maximum luminous efficiency of 1.95 cd A1. To the best of our knowledge, this is the first time green- and redemitting BNCMs have been reported; and the EL performance of our OLEDs is one of the best amongst spin-coated OLEDs reported so far. Furthermore, white light-emitting diodes (WLEDs) were fabricated with the green- and red-emitting BNCMs doped into a blue-emitting polymer host poly(9,9-dioctylfluorene- 2,7-diyl) (PFO) as EML, and a blend of poly[N,N0 -bis(4-butylphenyl)-N,N0 -bis(phenyl) benzidine] (poly-TPD) and poly(N-vinylcarbazole) (PVK) as hole-transporting layer (HTL).[41–43] By adjusting the weight ratio of BNCMs within the EML, high luminance as well as pure white light were achieved for the WLEDs.

S

2. Results and Discussion S N N

N

2.1. Synthesis

B(OH)2

The synthetic routes towards the four BNCMs are outlined in Scheme 1. Br TBBBT 0 0 dimethoxy-1,1 -binaphthyl- 3,30 - diyl2, 2 N N B(OH)2 S diboronic acid (1) and 2,20 -dimethoxy1,10 -binaphthyl-3-ylboronic acid (2) were synthesized according to literature known N procedures.[44] The synthesis of all BNCMs started from 4,7-dibromobenzoS N N [1,2,5]thiadiazole (3), which was syntheS B(OH)2 N N sized via bromination of benzo[1,2,5]thiadiazole using bromine and HBr under OMe + Br Br OMe MeO OMe reflux.[45] The reaction of 3 with tributylOMe MeO (thiophene-2-yl)stannane afforded 4 and 5 BBB based on Stille coupling reactions. 6 was S synthesized by further bromination [of 5] N N S N N B(OH) S (check in scheme) with N-bromosucciniS OMe + Br S Br mide (NBS),[46] while 7 was synthesized via S OMe OMe MeO a Heck reaction with a controlled feeding OMe MeO ratio of N,N-diphenyl-4-vinylbenzenamine BTBTB to 3.[47] Scheme 1. Synthesis routes of binaphthyl-containing molecules (BNCMs). TBT and TBBBT were synthesized by Suzuki coupling between 1 and 4 or 1 and hole-transporting units triphenylamine and thiophene long 7, and obtained in yields of 53.6% or 65.6%, respectively. BBB wavelength emissions in the range of green and red light were and BTBTB were synthesized by Suzuki coupling between 2 realized. Based on their amorphous features these BNCMs and 3 or 2 and 6, and obtained in yields of 32% or 59%, were directly spin-coated from their solutions and formed respectively. All molecular structures were verified by 1H and N

+

OMe OMe

OMe OMe NS N

2

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C NMR spectroscopy, MALDI-TOF-MS, and/or HRMS (see Experimental section). 2.2. Photophysical Properties

All four BNCMs are soluble in a variety of organic solvents and can be directly spin-coated on substrates to form films. For the purpose of comparison, the four BNCMs were divided into two groups according to the location of the binaphthyl moieties in the molecules: binaphthyl moieties in the center, noted as type A (TBT and TBBBT); and binaphthyl moieties on both sides, noted as type B (BBB and BTBTB). The absorption and photoluminescence (PL) spectra of the BNCMs in solution and as solid films are shown in Figure 1, whereby Figure 1a displays the absorption and PL spectra of type A molecules. Based on the absorption edges of TBT (497 nm) and TBBBT films (566 nm), the corresponding optical energy gaps (Egopt) were calculated to be 2.49 and 2.19 eV, respectively. For the individual molecules the absorption and emission bands of the solid films only experienced a little bathochromic-shift compared to those obtained in chloroform solution, with the main characteristics of the spectra remaining 1.2

TBT-film TBT-solution TBBBT-film 1.0 TBBBT-solution

(a)

Aborption (a.u.)

1.0 0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0

200

300

400

500

600

700

800

PL Intensity (a.u.)

1.2

900

Wavelength (nm) 1.2

BBB-film

(b)

1.2

BBB-solution

similar. This phenomenon indicates only weak intermolecular interaction within the BNCMs films. Our interpretation is that the highly twisted structure and the natural steric hindrance of the binaphthyl moiety help decrease the intermolecular interactions in the film state. The PL emission bands of TBT and TBBBT, located at ca. 533 nm and 607 nm, represent yellowish–green and reddish–orange emission, respectively. This difference in emission color between the two molecules originates from the much stronger intramolecular charge transfer effect of TBBBT compared to TBT. Similar results were obtained for the type B molecules, as evidenced by the absorption and PL spectra shown in Figure 1b. According to the absorption edges of BBB and BTBTB (463 nm and 622 nm, respectively), their corresponding Egopt were calculated to be 2.68 and 1.99 eV, respectively. The PL emission colors of BBB and BTBTB are bluish–green and red, respectively, as can be determined from their emission band peaks at 497 nm and 629 nm, respectively. It is worth mentioning that although their structures are similar (BTBTB has two thiophene moieties more than BBB), these two molecules show different color emissions. This difference can be interpreted as an enhancement of the effective conjugation range upon introduction of the two additional thiophene groups, and as the charge transfer taking place between the electron-rich thiophene rings and the electron-deficient benzothiadiazole. The PL quantum efficiencies (FPL) of the two types of molecules in solution are 0.41, 0.54, 0.10, and 0.48 for TBT, TBBBT, BBB and BTBTB,[48] respectively. BBB shows the lowest FPL value amongst the four molecules. The low FPL value of BBB may be attributed to its molecular structure. BBB is constructed of one benzothiadiazole that is sandwiched between two binaphthyl moieties. The single bond linkage between benzothiadiazole and binaphthyl allows the two binaphthyl moieties to twist more freely, which non-radiatively dissipates the energy of the excited-state, and hence leads to a decrease in fluorescence efficiency.[49] Except for BBB, the FPL values of the other three molecules are high enough for them to be promising candidates as emitters in OLEDs. In the following, the electrochemical and electroluminescence properties of TBT, TBBBT, and BTBTB are discussed.

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13

BTBTB-film 1.0

1.0

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 300

400

500

600

700

800

PL Intensity (a.u.)

Aborption (a.u.)

BTBTB-solution

0.8

900

Wavelength (nm) Figure 1. Absorption and photoluminescence (PL) spectra of binaphthylcontaining molecules in chloroform solution and in thin solid films. a) type A molecules, b) type B molecules.

Adv. Funct. Mater. 2008, 18, 3299–3306

2.3. Electrochemical Properties Cyclic voltammetry (CV) measurements were employed to investigate the electrochemical properties of the materials. The CV curves of TBT, TBBBT, and BTBTB are shown in Figure 2. All oxidation and reduction cycles of the film samples were measured in an acetonitrile solution of tetrabutylammonium hexafluorophosphate as the electrolyte. Sample films were coated onto the surface of a Pt working electrode by solutioncasting. It can be seen that TBT, TBBBT, and BTBTB show well-reversible oxidation and reduction processes, which indicates their high elecrochemical stability suitable for both p- and n-doping. This is a typical feature of triphenylamine and benzothiadiazole containing molecules. Relative to the Ag/Agþ electrode, the onset oxidation (Eoxonset) and reduction


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can be concluded that the energy levels and the energy gap of TBT match well with those of the two red-emitting molecules which is necessary for exciton energy transfer between them. Once a blue-emitter with suitable energy levels is chosen, white light emission should be easily realized using a blend of three RGB emitting materials.

Current (a.u.)

TBBBT

BTBTB

TBT

-2.5 -2.0 -1.5 -1.0 -0.5

2.4. Characterization of OLEDs

0.0

0.5

1.0

1.5

+

Potential (V vs Ag/Ag ) Figure 2. Cyclic voltammograms of TBBBT, BTBTB, and TBT films on a platinum electrode in a 0.1 M solution of Bu4NPF6 in acetonitrile.

potentials (Eredonset) of the materials were 0.87 V and 1.66 V for TBT, 0.54 V and 1.63 V for TBBBT, and 0.61 V and 1.27 V for BTBTB. From these values, Eoxonset and Eredonset, the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) and the band gap (EgCV) of the compounds were calculated using the equations listed below (Eqs. 1–3). onset HOMO ¼ e ðEox þ 4:71Þ½eV

(1)

onset þ 4:71 ½eV LUMO ¼ e Ered

(2)

onset onset EgCV ¼ e Eox Ered ½eV

(3)

Based on the stable amorphous properties of the BNCMs three OLEDs were fabricated using BNCM films as emittinglayers, with the films being directly spin-coated from their solutions. The device structure of the OLEDs was as follows: ITO/PEDOT:PSS (30 nm)/HTL (PVK:poly-TPD (1:1 wt/wt)) (40 nm)/EML (TBT, TBBBT, or BTBTB) (50 nm)/Ca (10 nm)/ Al (100 nm). The reasons for choosing a blend of PVK:polyTPD as hole-transporting layer are as follows: Firstly, polyTPD is resistant to non-polar organic solvents. Hence, during spin-coating of the EML onto the HTL, the HTL is barely dissolved because of the existence of poly-TPD, and a smooth interface can be formed, which greatly improves the device’s performance.[41,43] Secondly, considering the HOMO and LUMO energy levels of both, HTL and EML, the addition of the HTL between anode and EML greatly improves holeinjection, hole-transport, and electron-blocking abilities of the OLEDs (Fig. 3). Device characteristics, i.e., electroluminescence (EL), photoluminescence (PL) from the thin films, and Commission Internationale de L’Eclairage (CIE) coordinate values are summarized in Table 2 and Figure 4. As shown in Table 2, the OLED based on BTBTB exhibits a relatively low turn-on voltage (defined as the voltage required to give a luminance of 1 cd m2) of 2.2 V, a high maximum luminance of 8315 cd m2, and a high maximum luminescence efficiency of 1.95 cd A1. This is the first time spin-coated green- and -1.9 eV

Electrochemical potentials, energy levels, and band gaps are listed in Table 1. From Table 1 it can be seen that: i) for all three materials the band gaps calculated from the electrochemical measurements agree well with those obtained from the absorption spectra; ii) the band gaps of the two red-light emitting molecules TBBBT and BTBTB are both lower than the band gap of the green-light emitting molecule TBT; iii) the HOMO and LUMO energy levels of TBBBT and BTBTB are both located between those of TBT. From items ii) and iii) it

-2.3 eV

-3.05 eV -3.08 eV -3.44 eV ITO/PEDOT -5.0 eV

-5.1 eV

Table 1. Electrochemical onset potentials and electronic energy levels. -5.4 eV Molecule TBT TBBBT BTBTB

Eox

onset

0.87 0.54 0.61

Eredonset

HOMO [eV]

LUMO [eV]

EgCV

1.66 1.63 1.27

5.58 5.25 5.32

3.05 3.08 3.44

2.53 2.17 1.88

Egopt

[a]

2.49 2.19 1.99

[a] The optical band gap was calculated using the equation Egopt ¼ 1240 ledge1, where ledge is the onset value of the absorption spectrum in the direction of longer wavelengths.

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Ca -2.9 eV

HTL

-5.25 eV -5.32 eV -5.58 eV EML

Figure 3. Energy level diagrams of the investigated organic light-emitting diodes (OLEDs). In the hole-transporting layer (HTL) the solid and dashed lines correspond to poly(N-vinylcarbazole) (PVK) and poly[N,N0 -bis (4-butylphenyl)-N,N0 -bis(phenyl)benzidine] (poly-TPD), respectively. In the emitting layer (EML) the solid, dotted, and the dashed lines correspond to TBT, TBBBT, and BTBTB, respectively.


Adv. Funct. Mater. 2008, 18, 3299–3306


EML

Turn-on voltage [V]

TBT[a] TBT[b] TBBBT BTBTB

Maximum luminance [cd m2] at (voltage [V], current density [mA cm2])

4.4 2.5 3.6 2.2

737 7660 5085 8315

(13.7, (10.4, (13.8, (14.6,

Maximum EL efficiency [cd A1] at (voltage [V], current density [mA cm2])

1582.4) 1520.1) 1666.5) 1666.5)

0.1 1.7 0.37 1.95

(10.5, 265.1) (7.3, 59.8) (11.7, 372.8) (7.9, 59.2)

EL [nm]

PL [nm]

554 536 610 660

538 538 603 661

CIE coordinates

(0.47, (0.35, (0.57, (0.60,

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Table 2. Luminance characteristics of the OLEDs based on BNCMs.

0.46) 0.51) 0.39) 0.29)

[a] Hole-transporting layer (HTL) of the OLED: PVK/poly-TPD blend (1:1 wt/wt). [b] HTL of the OLED: PVK.

red-emitting BNCMs have been reported; and the EL performance of our OLEDs is one of the best amongst spincoated OLEDs reported so far. As shown in Figure 4, the EL and PL spectra of the TBBBT and BTBTB based OLEDs are nearly identical, whereas, in the case of TBT, the EL emission is red-shifted relative to its PL. In order to further investigate the cause of this phenomenon, an OLED with the following architecture was fabricated: ITO/ PEDOT:PSS (30 nm)/PVK (40 nm)/TBT (50 nm)/Ca (10 nm)/ Al (100 nm). With the HTL being formed by PVK alone, the EL (bold line) and PL spectra of TBT correspond quite well (Fig. 4). This result indicates that the red-shifted EL spectrum of the former OLED is probably due to an exciplex emission associated with poly-TPD and TBT, which can also be deduced from the energy levels shown in Figure 3. The energy difference between the HOMO of poly-TPD (5.1 eV) and the LUMO of TBT (3.05 eV) is 2.05 eV, which is lower than the bandgap of TBT. This could be the reason for the observed redshift in EL emission of the TBT based OLED with poly-TPD in the HTL. As shown in Table 2, the device characteristics of TBT are greatly improved when only PVK is used as HTL, and a more than ten-fold increase in maximum luminance and EL

1.0

EL of TBT EL of TBBBT EL of BTBTB

Intensity (a.u.)

0.8

PL of TBT PL of TBBBT PL of BTBTB

0.6

EL of TBT (HTL:PVK)

0.4 0.2 0.0 400

500

600

700

800

900

Wavelength (nm)

Figure 4. Electroluminescence (EL) and PL spectra of TBT, TBBBT, and BTBTB thin-films. The black bold line stands for the OLED with PVK as the HTL. Others stand for OLEDs with a blend of PVK and poly-TPD (1:1 wt/wt) as HTL.

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efficiency are observed. The corresponding CIE coordinate values of TBT are (0.35, 0.51), which corresponds to a yellowish–green emission. As for the devices based on TBBBT and BTBTB, their CIE coordinates are (0.57, 0.39) and (0.60, 0.29), respectively, corresponding to reddish–orange and red emissions. 2.5. Characterization of WLEDs White light-emitting diodes (WLEDs) were realized using the blue light-emitting polymer PFO as host material and the BNCMs as green- and red-emitting dopants. The reason we choose PFO is because of its high photoluminescence efficiency, good charge-transport characteristics, and good thermal stability.[50–52] PFO can function as a host material owing to its large band gap (HOMO ¼ 2.2 eV; LUMO ¼ 5.7 eV), thus green and red light can be realized through energy transfer and charge trapping processes from host to guest molecules. The structure of the WLEDs was ITO/ PEDOT:PSS (30 nm)/HTL (50 nm)/EML (60 nm)/Ca (15 nm)/ Al (100 nm). The HTL consisted of a blend of two hole transporting materials, PVK and poly-TPD (1:1 wt/wt). The EML was constructed by blending blue emitter (PFO), green emitter (TBT), and red emitter (TBBBT for device A and BTBTB for device B). Because the color of the emitting light greatly depends on the ingredient ratio of the blend materials in the EML, we explored different blend ratios in order to obtain pure white light. With respect to EL spectra and CIE values we obtained the following optimized weight ratios: PFO:TBT:TBBBT ¼ 1:1%:0.2% for device A and PFO:TBT:BTBTB ¼ 1:1%:0.08% for device B. The quantity of BTBTB needed is lower than that of TBBBT, which may be a consequence of the PL spectrum of TBT overlapping well with the absorption spectrum of BTBTB, thus the energy transfer from TBT to BTBTB being more efficient. The EL spectra of the optimized WLEDs are shown in Figure 5. It can be seen that both, the emission bands of device A as well as those of device B cover the entire visible range between 380 and 780 nm. When comparing the EL spectra of the devices with the PL spectra of the corresponding individual emitting materials, it can be seen that the blue-emitting band at around 430 nm and the green-emitting band at around 530 nm can be


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10000

EL of Device A PL of PFO PL of TBT PL of TBBBT

EL intensity / a.u.

0.8 0.6 0.4 0.2

1200

(a)

1000

device A device B

1000

2

(a)

800

100

600 400

10

200 1

0.0

0 0

350 400 450 500 550 600 650 700 750 800

2

4

6

Current Density (mA/cm )

1.0

Luminance ( cd/m2 )

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8

10

12

14

16

18

Bias (V)

Wavelength / nm 2.1

(b)

Luminance Efficiency (cd/A)

EL intensity / a.u.

1.0

EL of Device B PL of PFO PL of TBT PL of BTBTB

0.8 0.6 0.4 0.2 0.0

(b) device A device B

1.8 1.5 1.2 0.9 0.6 50

100

150

200

250

300

350

400

350 400 450 500 550 600 650 700 750 800 2

Current Density (mA/cm )

Wavelength / nm

Figure 5. EL spectra of the fabricated white light-emitting diodes (WLEDs). a) device A and b) device B. For comparison, the PL spectra of the corresponding individual emitting materials are also shown.

Figure 6. a) Current–voltage and luminance–voltage characteristics, and b) EL efficiency characteristics of the two WLEDs.

attributed to the emission of PFO and TBT, respectively, while the red-emitting bands at around 620 nm and 650 nm can be assigned to the emission of TBBBT (Fig. 5a) and BTBTB (Fig. 5b), respectively. The CIE coordinate values measured for the devices with optimized blend ratios are: (0.35, 0.32) for device A (at 10 V) and (0.35, 0.32) for device B (at 11 V). Both coordinate values a fairly close to the standard white light CIE coordinate value of (0.33, 0.33), which indicates that the devices emit pure white light. The CIE coordinate values of the optimized WLEDs were further investigated at different applied voltages. They vary slightly from (x ¼ 0.29, y ¼ 0.28) at 6.0 V to (x ¼ 0.36, y ¼ 0.33) at 12.0 V for device A, and from (x ¼ 0.33, y ¼ 0.28) at 7.0 V to (x ¼ 0.38, y ¼ 0.33) at 13.0 V for device B. This shows

that, within the listed voltage ranges, the CIE coordinate values of the two devices are close to that of pure white light. Having established the optimum blend ratios of emitters in the EMLs of the two WLEDs, we investigated the luminance characteristics of both WLEDs. Figure 6a shows the current density–voltage and luminance–voltage characteristics of the devices. As evident from the graphs, the turn-on voltage and maximum luminance are 3.6 V and 3997 cd m2, respectively, for device A, and 5.0 V and 4001 cd m2, respectively, for device B. Moreover, the maximum luminous efficiencies are 2.12 cd A1 and 1.88 cd A1 for devices A and B, respectively, as indicated in Figure 6b. Corresponding luminance characteristics of both devices are listed in Table 3.

Table 3. Optimized blend ratios of emitters in the emitting layer (EML) and luminance characteristics of corresponding WLEDs. Blend weight ratios in EML B[a]:G[b]:R[c]

Turn-on voltage [V]

Maximum EL efficiency [cd A1] at (voltage [V], current density [mA cm2])

Maximum luminance [cd m2] at (voltage [V], current density [mA cm2])

CIE coordinates at (voltage [V], current density [mA cm2])

Device A

1:1%:0.2%

3.6

2.12 (6.5, 15)

3997 (12.8, 833.3)

Device B

1:1%:0.08%

5.0

1.88 (9.9, 28)

4001 (16.4, 832.9)

(0.35, 0.32) (10, 238) (0.35, 0.32) (11, 61)

[a] Blue-emitter: PFO. [b] Green-emitter: TBT. [c] Red-emitter: TBBBT for device A, BTBTB for device B.

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Four binaphthyl-containing molecules (BNCMs), greenemitting BBB and TBT as well as red-emitting BTBTB and TBBBT, were synthesized. Being located either in the center or on both sides of the molecules, the binaphthyl moieties help to decrease intermolecular interactions in the film state, and thus result in amorphous materials. Amongst the four BNCMs, TBT, TBBBT, and BTBTB show high PL quantum efficiencies (FPL) of 0.41, 0.54, and 0.48, respectively, in solution. With these favorable properties, OLEDs were fabricated from the BNCMs whereby the EMLs were directly spin-coated from solutions of TBT, TBBBT, and BTBTB. The spin-coated OLEDs exhibited relatively low turn-on voltages, high maximum luminances and high maximum luminescence efficiencies. The OLED based on BTBTB, for example, shows a turnon voltage of 2.2 V, a maximum luminance of 8315 cd m2, and a maximum luminescence efficiency of 1.95 cd A1. Using the green-emitting TBT and the red-emitting TBBBT or BTBTB as dopants, two WLEDs were fabricated with PFO as the blue-emitting host. Under optimized conditions pure white light emission with a high maximum luminance of 4000 cd m2 was realized. The results indicate that BNCMs are promising organic EL molecules with application in solution-processible light-emitting diodes.

4. Experimental Synthesis: All reactions were carried out in argon atmosphere using Schlenk techniques. 1H and 13C-NMR spectra were recorded on a Bruker 400 NMR instrument. Reagents were obtained from Acros Co. and used as received. The starting materials 4,7-dibromobenzo [1,2,5]thiadiazole and (R)-2,20 -bismethoxy-1,10 -binaphthyl-3,30 -diboronic acid were prepared according to literature procedures [44,45]. All organic compounds of small molecular weight were purified by sublimation before use. TBT: Under Ar atmosphere, to a 250 mL two necked flask were added 4,7-dibromobenzo[1,2,5]thiadiazole (0.7364 g, 2.5 mmol), (R)-3-dihydroxyborane-2,20 -dimethoxy-1,10 -binaphthyl (BN-B(OH)2, 0.402 g, 1.0 mmol), Pd(PPh3)4 (140 mg), KOH (0.56 g, 10 mmol), tri(2-methyl)phenylphosphine (46 mg), and 18-crown-6 (20 mg). Freshly distilled tetrahydrofuran (THF, 45 mL) and H2O (30 mL) were injected to dissolve the above mixture. The reaction was kept at reflux temperature for 48 h, and then cooled to room temperature. The reaction mixture was extracted with dichloromethane, washed three times with brine, and then dried with unhydrated MgSO4. The resulting crude mixture was purified by column chromotography (silica gel, petroleum ether/CH2Cl2, 6: 1) which gave 400 mg TBT (yield 54%) as yellowish–green solid. 1H NMR (300 MHz, CDCl3, d): 8.24 (s, 1H, naphthyl), 8.10 (d, 2H, naphthyl), 7.87 (d, 2H, phenyl), 7.14– 7.41(broad, 8H), 3.13 (s, 3H, CH3); 13C NMR (CDCl3, d): 154.7, 154.4, 152.4, 139.4, 134.1, 132.1, 130.5, 130.4, 130.2, 128.4, 128.0, 127.6, 126.9, 125.8, 125.6, 125.5, 125.1, 77.4, 77.2, 77.0, 76.6; MS (MALDITOF, m/z): 745.7; HRMS: calcd for C42H26N4O2S4, 746.1; found, 746.1. TBBBT: The same procedure as for TBT was used. A red solid with a yield of 66% (207 mg) was obtained. 1H NMR (300 MHz, CDCl3, d): 8.31 (s, 2H), 8.01 (d, J ¼ 16.5 Hz, 2H), 7.94 (d, 4H), 7.7 (d, J ¼ 16.5 Hz, 2H), 7.53 (d, 4H), 7.57 (d, J ¼ 8.27 Hz, 2H), 7.28–7.47 (m, 12H), 7.03–7.15 (m, 18H), 3.20 (s, 6H); MS (MALDI-TOF, m/z): calcd for C74H52N6O2S2, 1121.4; found, 1121.4.

Adv. Funct. Mater. 2008, 18, 3299–3306

BBB: Under Ar atmosphere, to a 100 mL two necked flask were added (R)-3-dihydroxyborane-2,20 -dimethoxy-1,10 -binaphthyl (0.625 g, 1.74 mmol), 4,7-dibromobenzo[1,2,5]thiadiazole (0.256 g, 0.87 mmol), Pd(PPh3)4 (50 mg, 5% equiv.), tri(2-methyl)phenylphosphine (13 mg, 5% equiv.), and 18-crown-6 (10 mg). Freshly distilled THF (20 mL) was injected to dissolve the above mixture. Then a KOH solution (4.35M, 20 mL) was injected. The reaction mixture was refluxed for 48 h. Then it was cooled to room temperature, extracted with dichloromethane, washed in turns with 1 M HCl and brine, and then dried with unhydrated MgSO4. The resulting crude mixture was purified by column chromotography (SiO2, CH2Cl2) which gave 214 mg BBB as yellowish–green solid (yield 32%). m.p. 175 8C; 1H NMR (300 MHz, CDCl3, d): 8.3 (S, 2H), 8.0 (s, 4H), 7.95 (d, 2H), 7.87 (d, 2H), 7.47 (d, 2H), 7.42 (t, 2H), 7.32 (d, 8H), 7.22 (t, 2H), 3.85 (s, 6H), 3.1 (s, 6H); HRMS: calcd for C50H36N2O4S, 760.2; found, 760.2. BTBTB: The same procedure as for BBB was used. A red solid was obtained in 50% yield (200 mg). 1 H NMR (300 MHz, CDCl3, d): 8.33 (s, 2H), 8.2 (d, J ¼ 2.7 Hz, 2H), 8.02 (d, J ¼ 9.0 Hz, 2H), 7.88–7.93 (m, 6H), 7.78 (d, J ¼ 2.7 Hz, 2H), 7.48 (d, J ¼ 9.0 Hz, 2H), 7.39 (t, J ¼ 7.3 Hz, 2H), 7.33 (t, J ¼ 7.7 Hz, 2H), 7.21– 7.28 (m, 6H), 7.1 (d, J ¼ 8.4 Hz, 2H), 3.83 (s, 6H), 3.39 (s, 6H); GCT-MS: 924.0; HRMS: calcd for C58H40N2O4S3, 924.2150; found 924.2153. Measurements: Ultraviolet-visible (UV-vis) absorption spectra were recorded on a Hitachi U-3010 UV-vis spectrophotometer. Photoluminescence (PL) and electroluminescence (EL) spectra were obtained using a Hitachi F-4500 fluorescence spectrophotometer. Cyclic voltammograms were recorded on a Zahner IM6e electrochemical workstation, using a Pt disk coated with thin-films of the BNCMs as working electrode, a Pt wire as counter electrode, a Ag/Agþ electrode as reference electrode, and 0.1 M tetrabutylammonium hexafluorophosphate dissolved in acetonitrile as electrolyte solution. Fabrication and Characterization of OLEDs: The device configuration was: ITO (indium-tin-oxide)/PEDOT:PSS (poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate)) (30 nm)/HTL/EML/Ca (10 nm)/ Al (100 nm). The fabrication process of the devices was as follows: Firstly, PEDOT:PSS (Baytron-P, Bayer AG) was spin-coated on a precleaned and UV/ozone treated ITO substrate and dried in a vacuum oven. Secondly, the materials for the hole-transporting layer (HTL) were dissolved in chlorobenzene and spin-coated on top of the PEDOT:PSS layer. Thirdly, the materials for the emitting layer (EML) were dissolved in toluene and spin-coated on top of the HTL. At last, a layer of Ca capped with Al was thermally deposited through a shade mask at a pressure of ca. 5 105 Pa. The current–voltage–luminance characteristics were obtained using a computer-controlled Keithley 236 Source-Measure Unit and a Keithley 2000 Multimeter coupled with a Si photomultiplier tube. All measurements were performed at ambient conditions.

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3. Conclusions

Received: March 16, 2008 Revised: June 17, 2008

[1] W. Jones, in Organic Molecular Solids: Properties and Applications, (Ed: W. Jones), CRC Press, Boca Raton, FL 1998, p. 441. [2] K. Mu¨llen, G. Wegner, in Electronic Materials: The Oligomer Approach, (Eds: K. Mu¨llen, G. Wegner), Wiley-VCH, Weinheim 1998, p. 599. [3] C. W. Tang, S. A. VanSlyke, Appl. Phys. Lett. 1987, 51, 913. [4] U. Mitschke, P. Ba¨uerle, J. Mater. Chem. 2000, 10, 1471. [5] C. H. Chen, J. Shi, C. W. Tang, Macromol. Symp. 1997, 125, 1. [6] P. Strohriegel, J. V. Grazulevicius, Adv. Mater. 2002, 14, 1439. [7] S. Wang, W. J. Oldham, Jr., R. A. Hudack, G. C. Bazan, J. Am. Chem. Soc. 2000, 122, 5695.


www.afm-journal.de

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[8] M. D. Joswick, I. H. Campbell, N. N. Barashkov, J. P. Ferraris, J. Appl. Phys. 1996, 80, 2883. [9] Y. Shirota, J. Mater. Chem. 2000, 10, 1. [10] Y. Shirota, J. Mater. Chem. 2005, 15, 75. [11] L. X. Zheng, R. C. Urian, Y. Q. Liu, A. K. Y. Jen, L. Pu, Chem. Mater. 2000, 12, 13. [12] X. Z. Jiang, S. Liu, H. Ma, A. K. Y. Jen, Appl. Phys. Lett. 2000, 76, 1813. [13] L. Pu, Macromol. Rapid Commun. 2000, 21, 795. [14] X. W. Zhan, Y. Q. Liu, G. Yu, X. Wu, D. B. Zhu, R. G. Sun, D. K. Wang, A. J. Epstein, J. Mater. Chem. 2001, 11, 1606. [15] E. Diez-Barra, J. C. Garcia-Martinez, R. del Rey, J. Rodriguez-Lopez, F. Giacalone, J. L. Segura, N. Martin, J. Org. Chem. 2003, 68, 3178. [16] L. Z. Gong, L. Pu, Tetrahedron Lett. 2001, 42, 7337. [17] J. C. Ostrowski, R. A. Hudack, M. R. Robinson, S. J. Wang, G. C. Bazan, Chem. Eur. J. 2001, 7, 4500. [18] H. Benmansour, T. Shioya, Y. Sato, G. C. Bazan, Adv. Funct. Mater. 2003, 13, 883. [19] M. Berthod, G. Mignani, G. Woodward, M. Lemaire, Chem. Rev. 2005, 105, 1801. [20] M. Kubinyi, K. Pal, P. Baranyai, A. Grofcsik, I. Bitter, A. Grun, Chirality 2004, 16, 174. [21] J. Lin, Z. B. Li, H. C. Zhang, L. Pu, Tetrahedron Lett. 2004, 45, 103. [22] J. Lin, Q. S. Hu, M. H. Xu, L. Pu, J. Am. Chem. Soc. 2002, 124, 2088. [23] I. Kavenova, R. Holakovsky, M. Hovorka, V. Kriz, P. Anzenbacher, P. Matejka, V. Kral, J. W. Genge, J. L. Sessler, Chem. Listy 1998, 92, 147. [24] D. V. Gribkov, K. C. Hultzsch, Chem. Commun. 2004, 6, 730. [25] P. Kocovsky, S. Vyskocil, M. Smrcina, Chem. Rev. 2003, 103, 3213. [26] J. P. Genet, Acc. Chem. Res. 2003, 36, 908. [27] Y. Chen, S. Yekta, A. K. Yudin, Chem. Rev. 2003, 103, 3155. [28] R. A. Van Delden, T. Mecca, C. Rosini, B. L. Feringa, Chem. Eur. J. 2004, 10, 61. [29] Y. Yokoyama, T. Sagisaka, Chem. Lett. 1997, 8, 687. [30] Y. Yarovoy, A. Nunez, X. Luo, M. M. Labes, Mol. Cryst. Liq. Cryst. 1996, 289, 1. [31] G. Koeckelberghs, S. Sioncke, T. Verbiest, I. Van Severen, I. Picard, A. Persoons, C. Samyn, Macromolecules 2003, 36, 9736. [32] L. Di Bari, G. Pescitelli, P. Salvadori, J. Am. Chem. Soc. 1999, 121, 7998. [33] Q.-S. Hu, D. Vitharana, G.-Y. Liu, V. Jain, M. Wagaman, L. Zhang, T. R. Lee, L. Pu, Macromolecules 1996, 29, 1082. [34] Y. Meng, W. T. Slaven, D. Wang, T. J. Liu, H. F. Chow, C.-J. Li, Tetrahedron: Asymmetry 1998, 9, 3693. [35] R. Gomez, J. L. Segura, N. Martin, Chem. Commun. 1999, 619. [36] A. K.-Y. Jen, Y. Liu, Q.-S. Hu, L. Pu, Appl. Phys. Lett. 1999, 75, 3745. [37] Y. Liu, G. Yu, A. K.-Y. Jen, Q.-S. Hu, L. Pu, Macromol. Chem. Phys. 2002, 203, 37.

www.afm-journal.de

[38] X. W. Zhan, S. Wang, Y. Q. Liu, X. Wu, D. B. Zhu, Chem. Mater. 2003, 15, 1963. [39] Q. G. He, H. Z. Lin, Y. F. Weng, B. Zhang, Z. M. Wang, G. T. Lei, L. D. Wang, Y. Qiu, F. L. Bai, Adv. Funct. Mater. 2006, 16, 1343. [40] Q. C. Zhou, A. J. Qin, Q. G. He, G. T. Lei, L. D. Wang, Y. Qiu, C. Ye, F. Teng, F. L. Bai, J. Lumin. 2007, 122, 674. [41] Q. J. Sun, J. H. Hou, C. H. Yang, Y. F. Li, Appl. Phys. Lett. 2006, 89, 153501. [42] Q. J. Sun, B. H. Fan, Z. A. Tan, C. H. Yang, Y. F. Li, Y. Yang, Appl. Phys. Lett. 2006, 88, 163510. [43] Y. Zhou, Q. J. Sun, Z. A. Tan, H. Z. Zhong, C. H. Yang, Y. F. Li, J. Phys. Chem. C 2007, 111, 6862. [44] K. B. Simonsen, K. V. Gothelf, K. A. Jørgensen, J. Org. Chem. 1998, 63, 7536. [45] K. Pilgram, M. Zupan, R. J. Skiles, J. Heterocycl. Chem. 1970, 7, 629. [46] F. Huang, L. T. Hou, H. L. Shen, J. X. Jiang, F. Wang, H. Y. Zhen, Y. Cao, J. Mater. Chem. 2005, 15, 2499. [47] Q. G. He, C. He, Y. X. Sun, H. X. Wu, Y. F. Li, F. L. Bai, Thin Solid Films, in press DOI: 10.1016/j.tsf.2007.10.058. [48] Solutions for the measurement of fluorescence quantum yields were prepared in such concentrations that the maximum absorbance was approximately 0.05. The quantum yield is calculated using the following equation (Eq. 4):

2 Iu As nu Fu ¼ Fs Is Au ns

[49] [50] [51] [52]

ð4Þ

where the subscripts s and u stand for standard and unknown sample, Au and As are the absorbances of sample and standard at the excitation wavelength, Iu and Is are the integrated emission intensities of sample and standard, and nu and ns are the refractive indices of the corresponding solutions (pure solvents are assumed). The quantum yield values of TBT and BBB are reported relative to a solution of quinine sulfate in 0.1 M H2SO4, with the corresponding fluorescence quantum yield of the standard (Fs) being 0.58. The quantum yield values of TBBBT and BTBTB are reported relative to fluorescein solution in 0.1 M NaOH, with a corresponding Fs value of 0.95. Q. Peng, Y. P. Yi, Z. G. Shuai, J. S. Shao, J. Am. Chem. Soc. 2007, 129, 9333. U. Scherf, E. J. W. List, Adv. Mater. 2002, 14, 477. M. Gross, D. C. Mu¨ller, H.-G. Nothofer, U. Scherf, D. Neher, C. Bra¨uchle, K. Meerholz, Nature 2000, 405, 661. T. Miteva, A. Meisel, W. Knoll, H.-G. Nothofer, U. Scherf, D. C. Mu¨ller, K. Meerholz, A. Yasuda, D. Neher, Adv. Mater. 2001, 13, 565.


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